Ventilation is essential to maintain a healthy indoor environment. However, in summer and winter, thermal conditions of outdoor air are so poor that ventilation cannot but increase energy consumption. Thus, energy-efficient ventilation system is very important. Thermal labyrinth system is a ventilation system which intakes outdoor air through an underground concrete structure shaped like a labyrinth. By the heat exchange with ground, this system can precool and preheat the outdoor air in summer and winter, respectively. The aim of this study is to analyze the energy efficiency of thermal labyrinth system, applied to the E University building recently built in Seoul, in summer and autumn (from Aug. 1 to Nov. 30). As results, it was found that average cooling and dehumidifying effects of thermal labyrinth system in Aug. are $3.7^{circ}C$ and $3.1g_w/kg_{da}$ (9.92kW and 18.51kW), respectively, and average heating and humidifying effects in Nov. are $8.7^{circ}C$ and $0.7g_w/kg_{da}$ (19.50kW and 3.89kW), respectively. Total outdoor air sensible and latent, cooling and heating load was found to be reduced by 38.8% through the thermal labyrinth system.